The invention relates to AM stereo radio broadcast receivers and is particularly directed to a receiver using the Magnavox system recently announced as one of the systems of choice by the Federal Communications Commission In this system, the conventional amplitude modulated (AM) radio channel carries the L+R stereo signal so that a conventional monaural radio receives a compatible signal. The L-R stereo signal is transmitted as a phase modulation (PM) of the carrier. A subaudible pilot tone also phase modulates the carrier and its phase modulation is subsantially greate than of the L-R component. Since the conventional radio will not respond to the PM, it will not be affected thereby. However, if a limiter and PM detector are added to a conventional radio, the AM will be ignored in the added circuit and the PM can be recovered. Therefore the L-R information and pilot signal can be separately recovered. It is then only necessary to matrix the two channels to recover the stereo signals for reproduction in a stereo audio system. While the circuits disclosed herein are intended for use with the proposed Magnavox system, it is to be understood that the functions performed can be used with other proposed AM stereo systems.
My copending patent application Ser. No. 197,294 filed Oct. 15, 1980, is titled AM STEREO PHASE MODULATION DECODER and is assigned to the assignee of the present invention. A PM decoder is disclosed for use with the Magnavox system receiver.
My copending patent application Ser. No. 187,006 filed Sept. 15, 1980, is titled FULL WAVE AMPLITUDE MODULATION DETECTOR CIRCUIT and is assigned to the assignee of the present invention. It discloses a detector circuit primarily intended for use in AM stereo receivers and it has application to the present invention as will be described hereinafter.
Both of the above applications are incorporated herein by reference.
In conventional radio receiver design it has been found that a "tight" automatic gain control (AGC) will produce the same output signal for virtually all stations. This means that the user cannot distinguish a strong station from a weak one merely by listening. It is common practice to provide an AGC characteristic that will provide reasonably constant output but is "loose" enough to cause strong signals to sound louder than the weak ones. In an AM stereo receiver this means that the L+R signal level is variable. Since the L-R signal from the phase demodulator is constant, due to limiter action, these two signals are difficult to matrix. In most AM stereo receiver designs a "tight" AGC is employed so that the L+R signal is relatively constant. This means that the desired tuning characteristic is lost.